Global Navigation Satellite Systems (GNSS) such as the U.S. NAVSTAR Global Positioning System (GPS), the European Galileo positioning system, and the Russian GLONASS system are increasingly relied upon to precisely determine the geographic position of receiver equipment and to establish timing according to a reliable, accurate reference. Certain aspects of air traffic control systems have evolved to depend more on GPS navigation as conventional approaches, such as those relying on secondary surveillance radar (SSR), VHF omnidirectional range (VOR), and distance measuring equipment (DME), have seen a diminished role. For example, aircraft equipped for Automatic Dependent Surveillance-Broadcast (ADS-B) surveillance rely primarily on GPS signals to determine their position and then report their position to other aircraft and to ground radio stations. Another example of dependence on the GPS system is found in the financial industry, where institutions rely on the GPS system to maintain the accurate timing required for financial transactions. A wide range of GPS-equipped consumer products such as mobile phones, portable computers, and navigation devices have become ubiquitous and depend exclusively on the GPS system, or comparable GNSS satellite networks, for timing and navigation.
For a variety of reasons, however, GPS service is not completely reliable. Locations surrounded by obstructions (e.g., mountainous regions and “urban canyons”) may receive GPS signals only intermittently or not at all. GPS satellite equipment can potentially suffer outages, failure, or damage, and atmospheric conditions can potentially impact signal reception. In aviation, the ADS-B surveillance system is dependent on GPS for accurate, efficient, and continuous navigation and surveillance. Reliance on GPS or other satellite systems can result in common mode failures for GNSS receivers and the ADS-B surveillance system without sufficient backup. Ground-based radar systems can provide surveillance but not navigation to aircraft as they exist today. Other aviation position derivation devices such as VOR/DME have insufficient accuracy and coverage to support reliable and efficient aircraft separation.
Thus, it would be desirable to have a reliable alternative to GPS that can provide accurate navigation and timing over a wide geographic area and which is seamless with GPS to the end user.